1. Field of the Invention
The present invention relates to a thin film transistor array of a horizontal electronic field applying type, and more particularly, to a thin film transistor array of a horizontal electronic field applying type, which enhances brightness and a method for fabricating the same.
2. Discussion of the Related Art
Generally, liquid crystal display devices display an image by adjusting light transmittance of liquid crystals using an electronic field. These liquid crystal display devices are divided into a vertical electronic field applying type and a horizontal electronic field applying type according to directions of electronic fields driving liquid crystals.
A liquid crystal display device of a vertical electronic field applying type drives liquid crystals using vertical electronic fields formed between pixel electrodes and common electrodes disposed opposite to each other on upper and lower substrates. Such a liquid crystal display device has the advantage of a high aperture ratio, but has the disadvantage of a narrow viewing angle.
A liquid crystal display device of a horizontal electronic field applying type drives liquid crystals using horizontal electronic fields formed between pixel electrodes and common electrodes disposed in parallel on a lower substrate. Such a liquid crystal display device has the advantage of a wide viewing angle.
With reference to FIG. 1, a liquid crystal display device of a horizontal electronic field applying type includes a thin film transistor (TFT) array 10 and a color filter array 15, which are opposed to each other with liquid crystals 9 interposed therebetween.
The color filter array 15 includes a black matrix 3, color filters 5, and an overcoat layer 7, which are sequentially formed on an upper substrate 1. The black matrix 3 serves to prevent leakage of light and optical interference between the neighboring color filters 5. The color filters 5 include red (R), green (G), and blue (B) color filters, and thus allow the liquid crystal display device to display colors. The overcoat layer 7 serves to level the upper substrate 1, on which the black matrix 3 and the color filters 5 are formed.
The TFT array 10 includes gate lines 12 and data lines 14, which cross each other on a lower substrate 11 to define pixel regions, TFTs, which are respectively connected to the gate lines 12 and the data lines 14, pixel electrodes 18, which are respectively connected to the TFTs, common electrodes 19, which are parallel with the pixel electrodes 18, and common lines 16, which are respectively connected to the common electrodes 19.
The TFTs supply a data signal from the data line 14 to the pixel electrode 18 in response to a gate signal from the gate line 12. A horizontal electronic field is formed between the pixel electrode 18, to which the data signal is supplied through the TFT, and the common electrode 19, to which a reference voltage is supplied through the common line 16. The liquid crystals 9 are rotated by the horizontal electronic field. The rotating degree of the liquid crystals 9 is adjusted according to the data signal. As described above, the liquid crystal display device of the horizontal electronic field applying type varies transmittance of light transmitting the pixel regions by adjusting the rotating degree of the liquid crystals 9 using the horizontal electronic field, and thus displays an image.
An upper polarizing plate 2a and a lower polarizing plate 2b, which transmit light polarized at a specific direction, are respectively attached to the outer surfaces of the upper substrate 1 and the lower substrate 11. Generally, a transmission axis (x) of the upper polarizing plate 2a and a transmission axis (y) of the lower polarizing plate 2b are perpendicular to each other.
As described above, the liquid crystal display device of the horizontal electronic field applying type varies transmittance of light transmitting the pixel regions by adjusting the rotating degree of the liquid crystals using the horizontal electronic field, and thus displays an image. Here, the liquid crystals 9 driven by the horizontal electronic field can contribute to brightness of the liquid crystal display device, only when the major axes of the liquid crystals 9 are oblique to the transmission axes (x and y) of the upper and lower polarizing plates 2a and 2b. That is, light transmitting the liquid crystals 9, the major axes of which are parallel with the transmission axes (x and y) of the upper and lower polarizing plates 2a and 2b, cannot transmit the upper polarizing plate 2a, thus lowering brightness.
The above liquid crystal display device of the horizontal electronic field applying type has been developed toward the enhancement of an aperture ratio and a light transmittance by designing the common electrode 19 and the pixel electrode 18 in various methods. As one method, a structure, in which the common electrode 19 and the pixel electrode 18 are formed on the same layer with transparent conductive metals, was proposed. In this case, the common electrode 19 is connected to the common line 16 through a hole, and thus a reference voltage is supplied from the common line 16 to the common electrode 19. Further, the common line 16 is extended so as to overlap with portions of the pixel electrode 18 on condition that an insulating film is interposed therebetween, thereby forming storage capacitor (Cst). The storage capacitor (Cst) stably maintains a signal supplied to the pixel electrode 18. When a liquid crystal display device of a horizontal electronic field applying type including these storage capacitor (Cst) is driven, an electronic field formed between the storage capacitor (Cst) and the end of the common electrode 19 causes the major axes of the liquid crystals 9 to be driven in parallel with the transmission axes (x and y) of the upper and lower polarizing plates 2a and 2b. Light transmitting the liquid crystals 9, the major axes of which are parallel with the transmission axes (x and y) of the upper and lower polarizing plates 2a and 2b, cannot transmit the upper polarizing plate 2a, thus lowering the brightness of the liquid crystal display device.